Deserts' 15 markers

Page 1

Explain two or more causes of aridity (such as atmospheric pressure,

winds, conventionality and cold ocean currents). 15 marks …

An arid

area is defined an area which has less than 250mm of precipitation a year, a

low amount. The combination of the lack of precipitation and high temperatures,

especially in the day, causes high potential evapo-transpiration which clearly leads

to the water deficit. Ultimately, this lack of precipitation is the cause of

aridity in deserts.

The first

cause is global atmospheric circulation, where air from the equator rapidly

rises, due to the concentrated solar rays from the sun, leaving low pressure at

ground level. This dry air travels north and sound, but then cools at 30-20

latitude (over deserts like the Sahara), causing it to sink. This descending

air creates high pressure towards ground level, and the air becomes warmer due

to compression, allowing it to hold more moisture. This all prevents

evaporating water from rising, stopping clouds from forming which stops precipitation.

The lack of precipitation is what causes aridity but in some continental

isolation deserts, like the Central Sahara, it works together with another

cause of aridity, continentally.

This is

where moist coastal winds drop their precipitation before they reach the centre

of continents, causing those areas to be arid. These winds pick up the land’s

remaining moisture when travelling overland, increasing the level of aridity of

the land.

The rain

shadow of the Andes range causes Relief in the Atacama, making it arid. This is

where trade winds blow from the south east, but is forced to rise over

mountains due to their height. As the air rises it condenses, causing precipitation

on the windward (east) side of the Andes. This leaves dry air to descend on the

leeward side, which leaves that side without any precipitation, and therefore extremely

arid. But I think it’s important to remember that the effect of Cold Ocean

Currents’, works together with relief to make the Atacama desert Arid.

Page 2

Assess the role of

water in the formation of desert landforms. 15 marks …

There are two types of landforms, water erosion landforms and water

deposition landforms. Water erosion landforms are those caused by rapid surface

run off and high river discharge causing powerful erosion in high lying areas.

However, deposition features are found in low lying areas formed when water

slows down loosing its energy and depositing it’s sediment.

Wadis are steep sided dry river channels caused by water transportation.

They’re caused when heavy discharge due to rapid surface run off leads to

abrasion and extremely powerful hydraulic action. This creates steep sided

& wide river channels. Eventually the water loses its energy and deposits

its sediment on the river bed. However, larger wadis are relict features from

previous climates which had far more precipitation. Some of the water doesn’t

dry up before it reaches the opening of the wadis onto pediments or plains.

This causes the depositional feature, alluvial fans.

These are cone shaped deposits of sediment, which is graded with the coarsest

material closest to the mountain. In the Death Valley, these can be many

kilometres long and can join other alluvial fans to form bahadas. When heavy

precipitation falls the sediment coming down from the wadis is deposited when

the ephemeral river emerges onto lowland where there’s a sudden decrease in

gradient, slowing the water down. As the water is no longer trapped by the

steep sided walls’ of wadis’s, it spreads out over a wide area into numerous

distributaries, triggering rapid deposition in a cone shape.

I think it’s important to remember that today water doesn’t have much of

a role in forming desert landforms, but it has made a dramatic impact in past

climates, as is evident from the presence of Relict Hills. These include

Inselbergs, Mesas and Buttes. Inselbergs can often be several kilometres in

diameter like Ayers’ Rock in Australia.